Future-generation multimedia wireless communication systems under recent active study are required to additionally process and transmit various types of information including video and wireless data, beyond the traditional voice service. The wireless communication systems aim at reliable communication for a plurality of users irrespective of their locations and mobility. However, a wireless channel experiences a number of problems such as path loss, shadowing, fading, noise, a limited bandwidth, limited power of terminals, and interference between users. Other challenges faced in designing a wireless communication system include resource allocation, mobility issues related to fast changing physical channels, portability, security, and privacy.
When a transmission channel experiences deep fading, a receiver has difficulty in determining a transmitted signal unless another version or a replica of the transmitted signal is transmitted additionally. Resources corresponding to another version or a replica are called diversity and diversity is one of the most significant factors contributing to reliable transmission on a wireless channel. Use of the diversity can maximize data transmission capacity or data transmission reliability. A system that implements diversity by means of multiple Transmission (Tx) antennas and multiple Reception (Rx) antennas is called a Multiple Input Multiple Output (MIMO) system.
The MIMO system implements diversity by Space Frequency Block Code (SFBC), Space Time Block Code (STBC), Cyclic Delay Diversity (CDD), Frequency Switched Transmit Diversity (FSTD), Time Switched Transmit Diversity (TSTD), Precoding Vector Switching (PVS), Spatial Multiplexing (SM), etc.
One of systems that have been considered as promising successors to 3rd Generation (3G) systems is Orthogonal Frequency Division Multiplexing (OFDM) that can mitigate inter-symbol interference with low complexity. In OFDM, an input serial data stream is converted to N parallel data and transmitted on N orthogonal subcarriers. Orthogonality is maintained among the subcarriers in the frequency domain. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access scheme that achieves multiple access by independently allocating a part of available subcarriers to each user in a system using OFDM as a modulation scheme.
FIG. 1 illustrates a wireless communication system.
Referring to FIG. 1, the wireless communication system includes at least one Base Station (BS) 20. Each BS 20 provides communication service to a specific geographical area (generally called a cell) 20a, 20b or 20c. Each cell may further be divided into a plurality of areas (called sectors). A User Equipment (UE) 10 may be fixed or mobile. The term UE may be replaced with Mobile Station (MS), User Terminal (UT), Subscriber Station (SS), wireless device, Personal Digital Assistant (PDA), wireless modem, handheld device, etc. The BS 20 is generally a fixed station communicating with the UE 10 and the term BS is interchangeable with evolved Node B (eNB), Base Transceiver System (BTS), Access Point (AP), etc.
Downlink (DL) refers to communication from a BS to a UE and Uplink (UL) refers to communication from a UE to a BS. A transmitter may be a part of a BS and a receiver may be a part of a UE, on downlink, whereas the transmitter may be part of the UE and the receiver may be part of the BS, on uplink.
The wireless communication system may be any of a MIMO system, a Multiple Input Single Output (MISO) system, a Single Input Single Output (SISO) system, and a Single Input Multiple Output (SIMO) system. The MIMO system uses a plurality of Tx antennas and a plurality of Rx antennas. The MISO system uses a plurality of Tx antennas and a single Rx antenna. The SISO system uses a single Tx antenna and a single Rx antenna. The SIMO system uses a single Tx antenna and a plurality of Rx antennas.
Hereinbelow, a Tx antenna refers to a physical or logical antenna used for transmitting one signal or stream and an Rx antenna refers to a physical or logical antenna used for receiving one signal or stream.
A 3rd (Generation Partnership Project Long Term Evolution (3GPP LTE) system employs MIMO.
FIG. 2 illustrates the structure of a radio frame in the 3GPP LTE system.
Referring to FIG. 2, a radio frame is divided into 10 subframes, each subframe including two slots. The slots of a radio frame are numbered from 0 to 19. A unit time during which one subframe is transmitted is defined as Transmission Time Interval (TTI). A TTI may be considered to be a scheduling unit for data transmission. For example, one radio frame may be 10 ms long, one subframe may be 1 ms long, and one slot may be 0.5 ms long.
This radio frame structure is purely exemplary and thus the number of subframes in a radio frame or the number of slots in a subframe may vary.
FIG. 3 illustrates the structure of a resource grid for the duration of one uplink slot in the 3GPP LTE system.
Referring to FIG. 3, an uplink slot includes a plurality of OFDM symbols in time by NUL Resource Blocks (RBs) in frequency. An OFDM symbol represents one symbol period, also called a Single Carrier-Frequency Division Multiple Access (SC-FDMA) symbol or an OFDMA symbol or symbol period according to systems. An RB is a resource allocation unit including a plurality of subcarriers in the frequency domain. The number of RBs included in an uplink slot, NUL depends on an uplink bandwidth set for a cell. Each element of the resource grid is called a Resource Element (RE).
One RB includes 7×12 REs, that is, 7 OFDM symbols in the time domain by 12 subcarriers in the frequency domain, which is purely exemplary. Thus, the numbers of subcarriers and OFDM symbols in an RB are not limited to the above specific values. Rather, the number of OFDM symbols or the number of subcarriers in an RB may vary. The number of OFDM symbols may change according to a Cyclic Prefix (CP) length. For example, an uplink slot includes 7 OFDM symbols in case of a normal CP, whereas an uplink slot includes 6 OFDM symbols in case of an extended CP.
A resource grid may be configured for a downlink slot like the resource grid of an uplink slot in the 3GPP LTE system.
FIG. 4 illustrates a downlink subframe structure.
A downlink subframe includes two slots, each slot including 7 OFDM symbols in case of a normal CP. Up to three OFDM symbols at the start of the first slot in a downlink subframe are used for a control region to which control channels are allocated and the other OFDM symbols of the downlink subframe are used for a data region to which a Physical Downlink Shared Channel (PDSCH) is allocated. The PDSCH is a channel on which a BS transmits data to a UE.
A Physical Downlink Control Channel (PDCCH) delivers information about resource allocation (a DL grant) and a transport format for a Downlink Shared Channel (DL-SCH), resource allocation information (a UL grant) about an Uplink Shared Channel (UL-SCH), paging information of a Paging Channel (PCH), system information on the DL-SCH, resource allocation information about a higher-layer control message such as a Random Access Response transmitted on the PDSCH, a set of Transmission Power Control (TPC) commands for individual UEs of a UE group, Voice Over Internet Protocol (VoIP) activation information, etc. Control information transmitted on the above-described PDCCH is called Downlink Control Information (DCI).
Now, a detailed description will be given of downlink Reference Signals (RSs).
In the 3GPP LTE system, two types of downlink RSs are defined for unicast service, Common RS or cell-specific RS (CRS) and Dedicated RS or UE-specific RS (DRS).
CRS is an RS shared among all UEs within a cell, for use in acquisition of channel state information and handover measurement. DRS is an RS specific to a UE, for use in data demodulation. Thus it can be said that CRS is a cell-specific RS and DRS is a UE-specific RS.
A UE measures CRSs and transmits feedback information such as Channel Quality Information (CQI), a Precoding Matrix Indicator (PMI), and a Rank Indicator (RI) to a BS. Then the BS performs downlink frequency scheduling using the feedback information.
To transmit RSs to the UE, the BS allocates resources, taking into account the amount of radio resources to be allocated to the RSs, exclusive positions of CRSs and DRSs, the positions of a Synchronization Channel (SCH) and a Broadcast Channel (BCH), and the density of the DRSs.
If a relatively large amount of resources are allocated to RSs, high channel estimation performance can be achieved but data rate is relatively decreased. On the other hand, if a relatively small amount of resources are allocated to RSs, high data rate can be achieved but the result low RS density may cause degradation of channel estimation performance. Accordingly, efficient resource allocation to RSs, taking into account channel estimation and data rate is a critical factor to system performance.
Meanwhile, DRS is used only for data demodulation, whereas CRS is used for both channel information acquisition and data demodulation in the 3GPP LTE system. Especially, a CRS is transmitted in each subframe in a broadband, through each antenna port. For example, for 2Tx antennas in the BS, CRSs are transmitted respectively through antenna port 0 and antenna port 1. For 4Tx antennas in the BS, CRSs are transmitted respectively through antenna port 0 to antenna port 3.
FIG. 5 illustrates the structure of an uplink subframe in the 3GPP LTE system.
Referring to FIG. 5, an uplink subframe may be divided into a control region and a data region. A Physical Uplink Control Channel (PUCCH) including uplink control information is allocated to the control region. A Physical Uplink Shared Channel (PUSCH) including user data is allocated to the data region. In order to maintain a single carrier property, RBs allocated to a UE are contiguous in the frequency domain. That is, the UE cannot simultaneously transmit a PUCCH and a PUSCH.
A PUCCH for a UE is allocated to an RB pair in a subframe. The RBs of the RB pair occupy different subcarriers in two slots. Thus, the frequencies of the RBs of the RB pair allocated to the PUCCH are changed over a slot boundary. As the UE transmits uplink control information on different subcarriers with passage of time, a frequency diversity gain can be achieved. m is a position index indicating the frequency-domain logical position of an RB pair allocated to a PUCCH in a subframe.
Uplink control information transmitted on the PUCCH includes a Hybrid Automatic Repeat reQuest Acknowledgement/Negative Acknowledgement (HARQ ACK/NACK), a Channel Quality Indicator (CQI) indicating a downlink channel state, and a Scheduling Request (SR) requesting uplink radio resource allocation.
The PUSCH is mapped to a transport channel, Uplink Shared Channel (UL-SCH). Uplink data transmitted on the PUSCH may be a data block for a UL-SCH transmitted during a TTI, namely a transport block. The transport block may be user information. Or the uplink data may be multiplexed data. For example, control information multiplexed with data may include a CQI, a PMI, an HARQ ACK/NACK, an RI, etc. Or the uplink data may include control information only.
Meanwhile, a high data rate is required. The most basic and safe solution to the need for a high data rate is to increase a bandwidth.
However, frequency resources are saturated at present and various techniques are partially used in a broad frequency band. To secure a broad bandwidth to satisfy higher data rate requirements for this reason, the concept of designing each of scattered bands so as to meet basic requirements for operating an independent system and aggregating a plurality of bands into one system has been introduced. This concept is called Carrier Aggregation (CA). Each independent operable band is defined as a Component Carrier (CC).
CA is adopted in an LTE-Advanced (LTE-A) system as well as in the LTE system.
Carrier Aggregation
A CA system is a wireless communication system that configures a desired broad band by aggregating one or more carriers each having a narrower bandwidth than the broad band. The CA system is also called a multiple carrier system, a bandwidth aggregation system, etc. CA systems may be categorized into a contiguous CA system using contiguous carriers and a non-contiguous CA system using non-contiguous carriers. It should be understood that a multiple carrier system or a CA system covers both a contiguous CC case and a non-contiguous CC case in the following description.
A guard band may be interposed between carriers in the contiguous CA system. To ensure backward compatibility with a legacy system, each of one or more carriers that are aggregated may use a bandwidth defined in the legacy system. For example, the 3GPP LTE system supports 1.4, 3, 5, 10, 15 and 20 MHz. Alternatively, a broad band may be configured by defining a new bandwidth, instead of using the bandwidths of the legacy system.
A UE may transmit or receive one or more carriers according to its capabilities in the CA system.
FIG. 6 illustrates an example of communication on a single CC. This communication may be conducted in the LTE system.
Referring to FIG. 6, data transmission and reception are performed in a single downlink band and a single uplink band corresponding to the downlink band in a typical Frequency Division Duplex (FDD) wireless communication system. A BS and a UE transmit and receive data and/or control information that is scheduled on a subframe-by-subframe basis. The data is transmitted and received in the data region of an uplink/downlink subframe and the control information is transmitted and received in the control region of the uplink/downlink subframe. For transmission and reception of data and control information, the uplink/downlink subframe carries signals on various physical channels. While FIG. 7 is described mainly in the context of FDD, the same description is also applicable to a Time Division Duplex (TDD) system in which a radio frame is divided into uplink and downlink in time.
FIG. 7 illustrates an example of communication on multiple CCs. The communication may be performed in the LTE-A system. The LTE-A system uses CA or bandwidth aggregation by collecting a plurality of uplink/downlink frequency blocks to use a broader frequency band. Each frequency block is transmitted on a CC. In the specification, a CC may refer to a frequency block for CA or the center subcarrier of the frequency block. These two meanings are interchangeably used.
Referring to FIG. 7, a bandwidth of 100 MHz may be supported by aggregating 5 20-MHz CCs on uplink/downlink. The CCs may be contiguous or non-contiguous in the frequency domain. For example, the bandwidths of uplink CCs may be configured into 5 MHz (UL CC0)+20 MHz (UL CC1)+20 MHz (UL CC2)+20 MHz (UL CC3)+5 MHz (UL CC4). In addition, asymmetrical CA is also possible by configuring different numbers of uplink CCs and downlink CCs. Asymmetrical CA may take place due to a limited available frequency band or may be artificially implemented according to a network setting. For example, despite the existence of N CCs in a total system band, a frequency band that a specific UE can receive may be limited to M(<N) CCs. Various CA parameters may be configured cell-specifically, UE group-specifically, or UE-specifically.
While an uplink signal and a downlink signal are transmitted on one-to-one mapped CCs in the illustrated case of FIG. 7 by way of example, the number of actual CCs carrying signals may vary depending on a network setting or the type of the signals.
For instance, when a scheduling command is transmitted in DL CC1 on downlink, data corresponding to the scheduling command may be transmitted on another DL CC or UL CC. In addition, control information related to a DL CC may be transmitted in a specific UL CC on uplink irrespective of DL-UL CC mapping. DCI may be transmitted in a specific DL CC in a similar manner.
FIG. 8 is a block diagram referred to for describing a 3GPP LTE uplink access scheme, SC-FDMA.
For LTE uplink, SC-FDMA is adopted, which is similar to OFDM but reduces the power consumption and power amplifier cost of a portable terminal by Peak to Average Power Ratio (PAPR) reduction.
SC-FDMA is very similar to OFDM in that a signal is transmitted on subcarriers by Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT). SC-FDMA is also similar to OFDM in that a simple equalizer can be used in the frequency domain by using a guard interval (CP) against Inter-Symbol Interference (ISI) caused by multi-path fading. Compared to OFDM, SC-FDMA improves the power efficiency of a transmitter by reducing the PAPR of the transmitter by about 2 to 3 dB through additional unique techniques.
A problem encountered with a conventional OFDM transmitter is that signals on subcarriers along the frequency axis are converted to a time signal by IFFT. Since IFFT is a process of parallel executions of the same operation, it increases PAPR.
Referring to FIG. 8, in SC-FDMA, information is first subjected to Discrete Fourier Transform (DFT) in a DFT processor to solve the above problem. A subcarrier mapper 13 maps the DFT-spread signal (or the DFT-precoded signal in the same meaning) to subcarriers and an IFFT processor 14 converts the mapped signals to a time signal.
SC-FDMA outperforms OFDM in terms of transmission power efficiency because the PAPR of a time signal after IFFT is not increased much due to the correlation among DFT, subcarrier mapping, and IFFT.
That is, the transmission scheme of performing IFFT after DFT spreading is called SC-FDMA.
SC-FDMA is advantageous in that it is robust against multi-path fading channels due to a similar structure to OFDM and the problem of a PAPR increase encountered with IFFT in OFDM is radically solved. As a consequence, power amplifiers can be used efficiently. SC-FDMA is also called DFT spread OFDM (DFT-s-OFDM).
That is, SC-FDMA can reduce PAPR or Cubic Metric (CM). Furthermore, the non-linear distortion range of a power amplifier can be avoided by using SC-FDMA as a transmission scheme, thereby increasing the transmission power efficiency of a UE having limited power consumption. Accordingly, user throughput can be increased.
The 3GPP is actively working on standardization of LTE-A evolved from LTE. Although SC-FDMA-based techniques competed with OFDM-based techniques as in the standardization process of LTE, clustered-DFT-s-OFDM has been adopted, which allows non-contiguous resource allocation.
The LTE-A system will be described below in detail.
FIG. 9 is a block diagram referred to for describing clustered DFT-s-OFDM adopted as an uplink access scheme in the LTE-A standard.
The main feature of clustered DFT-s-OFDM lies in that it can flexibly cope with a frequency selective fading environment by enabling frequency selective resource allocation.
Compared to the conventional LTE uplink access scheme, SC-FMDA, clustered DFT-s-OFDM adopted as an LTE-A uplink access scheme allows non-contiguous resource allocation. Therefore, uplink transmission data may be partitioned into a plurality of clusters.
That is, while the LTE system maintains the single carrier property for uplink, the LTE-A system allows non-contiguous allocation of DFT-precoded data along the frequency axis or simultaneous transmission of a PUSCH or PUCCH.